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It means the query failed for some reason and returned a Boolean false instead of a query result resource. Therefore you need to debug your query and also consider adding some defensive coding, so that you only call mysql_num_rows() if the query did not fail (i.e. is not false).

You might try running the query in phpMyadmin to make sure the query returns the right data. Is the conn.php connecting ok?

It is not a good practice to store the password in a readable format. It is best to encrypt the password. That way if your site is hacked the passwords will not be stolen along with usernames. When encrypted you just re-encrypt the login and compare encrypted to encrypted.